Ž .Fluid Phase Equilibria 158–160 1999 829–834

Refinery crude column overhead corrosion control, amine neutralizerelectrolyte thermodynamics, thermochemical properties and phase

equilibria

Diego P. Valenzuela, Ashok K. Dewan )

Equilon Enterprises LLC, Westhollow Technology Center, Post Box 1380, Houston, TX, 77251-1380, USA

Received 11 May 1998; accepted 3 January 1999

Abstract

Organic neutralizing amines are commonly used to combat corrosion in refinery crude column overheadsystems. Yet under-deposit corrosion and active acid corrosion are frequently reported in such refinery processunits. Several of these failures can be traced to poor application, or misuse, of organic neutralizing amines. Thecommon cause is usually a lack of understanding of the electrolytic thermodynamics, thermochemical properties

Ž .andror phase equilibria of these organic amines. To understand the root cause s of these failures, a rigorous,fundamental, thermo-dynamic approach was developed in Shell Oil. Important thermochemical and physicalproperties were identified and phase equilibria relevant to a typical crude unit overhead were outlined.Custom-developed apparatus and experimental procedures were used to measure the needed data and to validatethermodynamic consistencies. These methodologies and data generation techniques will be described here. The

w Ž .incorporation of these data into the frame work of commercial flowsheeters such as ProVision electrolytesw Ž w .and Aspen Plus with OLI Engine will also be discussed. The use of commercial flowsheeters is the

recommended way to practice our rigorous technology in the field. Complementary forms of output such asCharts, Nomographs, Spreadsheets and Phase Diagrams can also be produced and used by field staff. Tosuccessfully apply this technology to refinery crude column overheads in Shell and non-Shell crude towers, weset up a Technology Partnership Program, with a chemical supplier—Baker Petrolite. This relationship has

Žproved to be very successful in the USA and will be outlined. ‘ProVision’, ‘AspenPlus’ and ‘OLI Engine’ are.registered trademarks of Simulation Sciences, Aspen Technology and OLI Systems. q 1999 Elsevier Science

B.V. All rights reserved.

Keywords: Chemical equilibria; Gibbs energy; Corrosion control; Amine neutralizer; Thermochemical properties; Phaseequilibria

) Corresponding author. Tel.: q1-2815447664; fax: q1-2815448123; e-mail: akdewan@equilon.com

0378-3812r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.Ž .PII: S0378-3812 99 00067-9

( )D.P. Valenzuela, A.K. DewanrFluid Phase Equilibria 158–160 1999 829–834830

1. Summary

Ž .Organic neutralizing alkyl, alkanol, alkoxy and cyclic ether amines are commonly used to controlcorrosion in crude unit overhead systems. Yet under-deposit corrosion and active acid corrosion nearthe aqueous dew point are frequently reported to occur in such refinery process units. A significantnumber of the corrosion-related failures in crude unit overhead systems are the result of poorapplication of organic neutralizing amines. The misapplication usually stems from a lack ofunderstanding of the physical properties and phase equilibrium of these organic amines.

To better address the root cause of these failures, pertinent physical properties and the phasebehavior of organic amines and their hydrochloric salts were measured, screened and evaluated.State-of-the-art laboratory equipment and techniques were developed to collect the physical properties

Žand phase equilibrium data needed to predict the behavior of systems involving water, organic or,.inorganic salts, acid gases and organic amines used for corrosion control. In particular, the

Ž .thermodynamic state functions i.e., Gibbs Free Energy, Enthalpy and Entropy changes and physicalŽ .properties e.g., activity coefficients of dilute amine solutions, and the formation potential of amine

hydrochloride salts, were systematically determined.In the field, the data can be delivered and used in a multitude of ways. The popular forms are

Charts, Nomographs, Spreadsheets, Phase Diagrams, etc. The more rigorous approach involves thew 1 Ž .use of electrolyte process flowsheeters such as Simulation Sciences’ PROVISION electrolytes ,

or Aspen Technology’s Aspen Plusw process flowsheeters, to predict salt formation and corrosiveconditions in pipes and refinery process units.

In order to apply the rigorously developed technology most effectively to crude column overheadsystems, a Technology Partnership arrangement was defined between Shell Oil Products and achemical supplier—Baker Petrolite. As part of this arrangement, this newly developed basic thermo-dynamic data and methods were coupled to a customized commercial flowsheeter. Baker Petrolite, achemical vendor specializing in corrosion inhibitors and neutralizers, now routinely uses this

Ž .technology to provide Shell and non-Shell customers advice on types of additives, neutralizerdosage and injection points in crude column overhead systems.

2. Introduction

Ž .The presence of alkaline-earth magnesium, calcium, etc. chloride impurities in the crude oilsupplied to refineries introduces hydrochloric acid in the tops of crude columns. In a typical crudecolumn, due to the presence of stripping steam in the column, or water wash in an upstream DesalterUnit, or due to the presence of water in the supplied crude oil, hydrolysis of these alkaline-earthchlorides occurs and releases hydrochloric acid. For example,

MgCl q2H Os2HClqMg OHŽ .22 2

While magnesium and calcium chloride salts tend to hydrolyze in the feed heater and bottomsection of the crude column, the sodium chloride salt resists hydrolysis and is stable. The HCl vapors

1 Aspen Plusw is a registered trademark of Aspen Technology, and PROVISIONw is a registered trademark of SimulationSciences.

( )D.P. Valenzuela, A.K. DewanrFluid Phase Equilibria 158–160 1999 829–834 831

are volatile and exit the crude column along with the column overhead vapor. If untreated, the HClvapor will condense in the vicinity of the aqueous dew point in the crude column overhead system,causing active acid corrosion by the following overall reaction:

Feq2HClsFeCl qH2 2

Atomic hydrogen can form blisters in the metal wall and iron dissolves in the free water phaseproduced in the crude column overhead system.

Organic amines are added to the column overhead system to raise the pH at the aqueous dew pointand all points downstream of the aqueous dew point. It is practically impossible to measure the pH ofthe first water droplet produced. Hence the traditional practice in industry has been to overdose theneutralizer amine added to the crude column overhead. For amine hydrochloride salts having limitedmiscibility in water at the tube skin temperatures in the overhead exchanger bundles, this excess maycause solid amine salt to drop out by the following reaction:

R-NH g qHCl g sR-NH Cl sŽ . Ž . Ž .2 3

This reaction may occur above or below the aqueous dew point.If the temperature at which the above reaction occurs is higher than the melting point of the salt,

then we make no distinction between the aqueous dew point and the salt formation temperature:molten salts are never dry, as long as there is even a very small amount of water in the overheadvapors. In the flowsheeter, when molten salts are formed, there is some subcooling before anindependent free water phase can coexist. The wet salts, either solid or molten, are known to be veryaggressive in corrosion.

To provide an effective corrosion control program, adequate pH levels must be maintainedthroughout the crude column overhead system while avoiding the deposition of corrosive aminehydrochloride salts. The open literature provides limited phase diagrams for ammonia and ammonium

Ž .chloride deposition. Very little if any data are available for organic amines. The effort within ShellOil Products has focused on generation of the thermochemical properties and phase equilibrium datafor neutralizer amines and amine hydrochloride salts.

3. Corrosion control in crude column overheads

The use of filming inhibitors andror neutralizer amines to control corrosion in refinery crudecolumn overhead systems is a common refinery practice. Traditionally, ammonia has been used forthis service for many years. In the last two decades, the use of organic neutralizing amines has

Žbecome increasingly popular. Typically, the neutralizer amine is added to the overhead vapor or,. Ž .sometimes to the reflux returning to the column in very small dosage s . Often a single injection

point is used to introduce the chemical into the overhead system. Overhead systems with multipletrains of condensers and reflux drums require a more elaborate, multipoint injection system.

The design of a crude column overhead system corrosion control program must address thefollowing items:

Ž .1. Single or, Multiple Drums2. The pH at the Aqueous Dew point and pH in Drums3. Other SloprTramp Feeds to the Crude Column

( )D.P. Valenzuela, A.K. DewanrFluid Phase Equilibria 158–160 1999 829–834832

4. Rate of Neutralizer Injection5. Temperatures at Injection Point, Bulk Process and Skin Temperatures6. Amine Hydrochloride Salt Formation7. Hydrocarbon Fluid Flow8. Injection System Design9. Compatibility of Neutralizer and Filming Inhibitor

All of the above items are important for the successful use of neutralizers in crude columnoverhead systems. The practice is to pick a suitable neutralizer chemical that will raise the aqueousdew point pH sufficiently while preventing amine hydrochloride salt formation above the aqueousdew point. This will allow the minimization of acid corrosion in the vicinity of the aqueous dew pointand under-deposit salt corrosion above the aqueous dew point.

4. Thermodynamic framework

Phase and chemical equilibria calculations are done in commercial flowsheeters by specializedsubroutines accessing databanks containing data and parameters for a variety of species. Theinformation stored in these banks should be comprehensive enough to allow reliable prediction ofphase and chemical equilibria involving a variety of species in a broad range of temperature, pressureand composition. Given all these requirements, the amount of raw data needed to be stored canbecome too large to be practical. However, thermodynamics provides us with a framework thatensures self-consistent results, minimizes experimental effort, reduces the amount of information to bestored, and provides reliable ways of extrapolating and interpolating experimental data.

Classical thermodynamics establishes the phase equilibrium criteria as the equality of partial molarGibbs Free Energy, G, of every chemical species in every phase. For reactive systems, therequirement is that the stoichiometric sum of the partial molar Gibbs Free Energy of the reactants andproducts should be zero for every chemical reaction occurring in the system. Consequently, ourcapacity to predict phase and chemical equilibrium is determined, to a large extent, by our ability tocompute the partial molar Gibbs Free Energy of every species, in every phase, in the target system.

Ž .Thermodynamic path calculations e.g., gas to liquid, liquid to aqueous, aqueous to crystal areperformed to obtain the Gibbs Free Energy in each phase. Mathematical details of thermodynamicpath calculations are omitted to keep the length of this paper manageable.

5. Reduction to practice

The reduction to practice from a fundamental approach to tangible results that can be used in therefinery, is best done using customized commercial flowsheeters. This allows the practicing engineerto retain the rigor of the methodology while obtaining simulation results quickly and easily. Foreveryday use, several traditional forms of complementary output can be produced that allow field staffto examine tentative cases without having to run the commercial flowsheeters.

5.1. Charts

ŽCharts are usually defined to depict various operating condition changes in the field e.g., partial.pressures of hydrocarbons and water within an overhead tube bundle .

( )D.P. Valenzuela, A.K. DewanrFluid Phase Equilibria 158–160 1999 829–834 833

5.2. Nomographs

The traditional method of pH control in crude column overhead systems is to monitor the bootwater pH in the refinery and adjust the neutralizer dosage based on field measurements in theaccumulator boot. This approach suffers from a critical shortcoming. The pH at the aqueous dewpoint is only implicitly related to the accumulator boot water pH. We have found that the accumulatorboot water pH can often be in an acceptable range of 5.5 to 6.0 while the aqueous dew point pH is

Ž .unacceptably low i.e., less than 4.0 for carbon steel . From our rigorous model work, we candetermine the optimum dosage for achieving an acceptable pH across the entire overhead system.Specifically, the minimum amount of neutralizer needed to achieve an acceptable aqueous dew pointpH can be determined as a function of both chloride and ammonia content in the boot water.

5.3. Spreadsheets

With the advent of Microsoft Excelw 2 and Lotus-123w spreadsheets, process engineers havebecome very versatile in observing trends and analyzing data by changing one variable at a time andstudying the impact of that change. Two forms of spreadsheets are easily produced from our rigorousapproach. The first form of spreadsheet is designed for thermodynamic consistency checking ofthermochemical properties. As different thermodynamic pathways are measured in the laboratory andtheir data collated, they are entered into the spreadsheet. Individual thermodynamic pathways can benumerically added to see if there is data closure. It also serves as a convenient vehicle to transmit datato other developers who build thermodynamic data banks and customize flowsheeters for morerigorous use.

The second spreadsheet is oriented more for the practicing engineer doing ‘what if’ aminehydrochloride salt formation studies. By entering process conditions at a given point in the overheadsystem, the engineer can then compare salting tendencies for various amine hydrochlorides anddetermine which is best suited for the application. A new variable, ‘Salt Delta T ’, is defined for thispurpose. If this variable is positive, there is no salting tendency; a value of zero is the incipient saltformation point; a negative value flags salt formation. In practice, a fuzzy boundary around zero isused to delineate the incipient salt point.

5.4. Phase diagrams

The Phase Diagram for Ammonium Chloride Salt serves as an example of another form of outputwhich a refinery chemist might find useful.

5.5. Computer programs

Ž Ž . Ž . .Several computer programs such as ESP, CSP, ProVision e Hysim e , Aspen Plus are commer-cially available from commercial flowsheeter vendors. The choice of computer program is governedby the end-user’s preference for a given tool and his familiarity and dexterity in using the same.

2 Excelw is a registered trademark of Microsoft, and Lotus-123w is registered trademark of Lotus.

( )D.P. Valenzuela, A.K. DewanrFluid Phase Equilibria 158–160 1999 829–834834

6. Conclusions and future work

Traditional methods used to control corrosion in refinery crude column overhead systems areinadequate. Depending solely on boot water pH and analyses of a few ions in solution are a potentialtrap for the practicing engineers. Acceptable pH readings in the boot water can provide a false senseof security, while the overhead system has active acid corrosion at the aqueous dew point.

Neutralizer organic amines are often misapplied in the refinery due to a lack of understanding ofthe physical properties, thermochemical behavior and phase equilibria of amines and their hydrochlo-ride salts. Excess dosage of these neutralizers causes under-deposit corrosion above the aqueous dewpoint and is frequently the cause of failure in overhead systems.

It is almost impossible to measure the pH of the first water droplet at the aqueous dew point. Ithappens to be the most aggressive point for acid corrosion. A rigorous thermodynamic model, such asour work, can bridge this gap.

There is a severe paucity of data for organic amines and their hydrochloric acid salts. We havesuccessfully developed the Thermodynamic Framework and needed experimental data to rigorouslymodel crude column overhead units via customized commercial flowsheeters. Alternate forms ofoutput such as Charts, Nomographs and Spreadsheets can be used to conduct screening studies.

The developed fundamental methodology is thermodynamically consistent, rigorous and allowsinterpolationrextrapolation over a wide range of crude column tops conditions. Besides studying theuse of neutralizer chemicals, it can also be used to investigate water washing of tube bundles as an

Žalternate corrosion mitigation solution. Various water sources once-through water feeds, or recycle.water feeds can be simulated to determine the most optimum configuration for a particular refinery.

The development of a Technology Partnership between Shell Oil Products and a chemical suppliersuch as Baker Petrolite, allows us to make the most optimum and timely use of our rigorous

Ž . ŽThermodynamic Framework for Shell and non-Shell crude column overhead systems. This technol-.ogy has been successfully applied to over 30 refinery crude unit tops in North America.